Field of the Invention
The present invention relates to a semiconductive crystal containing n-type or p-type impurities. More particularly, the present invention relates to a semiconductive ZnSe single crystal to which n-type or p-type impurities are selectively doped and which is used in photoelectronic devices.
ZnSe is one of the II/VI group compound semiconductors and has the direct transition type band gap (E.sub.g) of as large as 2.7 eV. Therefore, ZnSe is a highly promising material of blue light emitting diode.
However, the blue light emitting diode has not been made from ZnSe, because no p-n junction can be formed and no good bulk substrate is produced.
Although the present invention does not relate to a method for producing the bulk single crystal, the present status of the bulk single crystal will be first explained since the nature of the bulk single crystal has large influence on the impurity doping.
The reason why it is difficult to produce the bulk single crystal of ZnSe is that ZnSe is easily sublimated and cannot be melted by simply heating it under atmospheric pressure so that it is difficult to grow the single crystal from a ZnSe melt.
Hitherto, several methods such as a high pressure melting method, an iodine transporting method, a solution growth method and a sublimation method have been proposed to grow the ZnSe single crystal. The first two methods can produce a comparatively large single crystal with good reproducibility, although the single crystal contains a large amount of impurities and many defects such as stacking faults and twin crystals. The last two methods cannot produce any large size single crystal. In addition, none of these methods can produce a single crystal having satisfactory sizes and characteristics.
The ZnSe single crystal produced by these method is a non-doped type one, but the single crystal as grown has a considerably high resistivity of about 10.sup.8 ohm.cm in the form of an n-type.
When such single crystal is used as a substrate of a light emitting diode (LED), one of electrodes is attached to one surface of the substrate and an exciting current flows through the substrate. Therefore, the substrate should not have high resistance. If the substrate had high resistance, a sufficient amount of electric current could not flow through the p-n junction.
To decrease the resistivity of the ZnSe single crystal, it is contemplated to dope an n-type or p-type impurity to the single crystal to make an n-type or p-type semiconductor. However, the amount of a dopant cannot be regulated in the ZnSe single crystal, while it is possible to regulate the amount of the dopant in the Si and GaAs single crystals. Reproducibility of the doping of ZnSe single crystal is not good. In addition, it has been believed that any p-type ZnSe single crystal could be produced.
To decrease the resistance of the ZnSe single crystal, proposed is a method comprising adding a ZnSe single crystal in a zinc melt or a zinc melt containing a suitable impurity and heating the melt at a temperature around 1,000.degree. C. This process intends to decrease the resistivity of the ZnSe single crystal from 10.sup.8 ohm.cm to 10.sup.1 ohm.cm by doping the n-type impurity. In this method, the impurity is diffused in the ZnSe single crystal in a thermal equilibrium state.
By the thermal equilibrium doping, the doping amount can be well controlled with good reproducibility in cases of Si and GaAs. That is, in cases of Si and GaAs, the concentration of the doped impurity and the resistivity can be freely adjusted.
However, in case of the ZnSe single crystal, the impurity is not adequately doped, so that the doped amount of the impurity cannot be determined by the concentration of the impurity in the zinc melt. This is because the ZnSe single crystal contains a large number of vacant lattices and impurities. Then, the impurity to be doped is not necessarily positioned at an intended site, and the originally present vacant lattices and/or impurities may be moved by the thermal treatment.
The p-type doping of the ZnSe single crystal is more difficult than the n-type doping of the ZnSe single crystal described above. Recently, the growth of the p-type ZnSe bulk single crystal through Li-doping by the vapor pressure controlling method was reported by J. Nishizawa et al., "Blue Light Emission from ZnSe p-n junction", J. Appl. Phys., 57 (6), 2210-2216 (1985). But the reported method has doubtful reproducibility.
Since the thermal equilibrium doping achieves unsatisfactory effects, many methods for ion implantation of impurities into the ZnSe single crystal have been proposed. The ion implantation is a non-equilibrium doping. Since the ions of impurity are forced to be implanted, the impurity ions can reach to any depth of the single crystal.
The ion implantation is one of successful doping methods in cases of Si and GaAs, and advantageously applied to a wide variety of ions. However, since the ion implantation causes distortion of the lattices, the ion implanted single crystals should be annealed. The ZnSe single crystal should be also annealed after ion implantation. However, since the impurities and/or the vacant lattices would be moved, the predetermined doping level is not achieved. That is, the doping of ZnSe single crystal cannot be controlled.
As explained above, by the thermal equilibrium doping, the kind and amount of the impurities cannot freely controlled. This can be attributed to the nature of the non-doped single crystal. The as grown ZnSe single crystal produced by the high pressure melting method or the iodine transporting method inherently contains a large amount of the impurities, and the state of the impurities are not stable in the crystal so that it may vary at high temperatures. If the ZnSe single crystal inherently containing the large amount of the impurities is thermally treated in the Zn melt for a long time, such impurities are activated. Although the impurities contained in the Zn melt diffuse into the single crystal, they will not occupy the intended lattice sites because of the presence of turbulence of the lattice structure.
Consequently, none of the known methods for doping the impurities in the single crystal can dope the ZnSe single crystal which is produced by the conventional methods with the impurity while freely selecting the kind and the amount of the impurity.